Optical measurements of the thickness of thin films, as well as processing of small pattern features, require an optical system operation with shorter wavelengths. The most traditional technique is limited by DUV (Deep Ultra Violet) spectral range (down to 190 nm, or, due to some sources, to 200 nm). The use of shorter wavelengths enables many metrology advantages for thin film applications, but suffers from strong absorption of light by air (and water).
The above problem can be overcome by operating with such short wavelengths under vacuum conditions (the so-called “Vacuum UV” or “VUV”) or in the environment of VUV non-absorbing gas like Nitrogen. For example, the VUV ellipsometer, commercially available from J. A. Woollam Co., Inc., operates in the spectral range down to 150 nm. The configuration of this system requires both an article under measurements and an optical system to be located within a sealed enclosure filled by Nitrogen.
This technique, however, is unsuitable for integrated metrology systems, because of the need for a small system size and high throughput. Indeed, such procedures as loading and unloading of an article (wafer) to and from such a sealed enclosure are time consuming, requiring sealing/unsealing of a processing tool, while maintaining a required gas pressure inside.